The availability of antibodies that bind to the cyclic nucleotides with high specificity, has made it possible to apply immunohistochemical techniques to the study of the localization of the cyclic nucleotide in tissues and cells. Of importance may be the distinct pattern of binding of the cyclic nucleotides to cellular organelles, and the relationship of this binding to the regulation of cellular growth. We are presently studying the binding patterns of cyclic nucleotides by the immunohistochemical techniques in growing and regressing dimethylbenzanthracene induced mammary carcinomas. Regression is induced by either oophorectomy or administration of dibutyryl cyclic adenosine monophosphate. In addition we are quantitating cyclic nucleotide binding capacity with tritiated cyclic nucleotides, and determining cyclic nucleotide dependent protein kinase activity in different subcellular organelles in an attempt to correlate the quantitative data with our immunohistochemical observations.